Aside from those systems, which cure under the action of UV by a free radical polymerization, systems containing cationically curable vinyl compounds have been developed in recent years, for which the curing is initiated, in particular, by diaryliodonium and triarylsulfonium salts. The advantage of the cationically curing systems lies in the insensitivity of the curing reaction to the effects of oxygen of the air, the rapid formation of a film and the nonpolluting nature of these systems.
The cationic curing of vinyl monomers is described in U.S. Pat. Nos. 4,617,238, 4,518,788 and 4,705,887.
For such a curing reaction, UV-curable vinyl ether compounds have gained special attention because they are cured rapidly and processed economically and they do not contaminate the environment. Such vinyl ether compounds can be synthesized in various ways. In a survey article of the "Rate-Determining Factors in Cationic UV Curing" in the Journal, Farbe und Lack, 1987, pages 803 to 807, the following possible syntheses are given:
1. Base-catalyzed addition reaction of Reppe between acetylene and diols ##STR2##
2. Phase transfer-catalyzed condensation of 2-chloroethyl vinyl ether and diols ##STR3##
3. Catalyzed rearrangement of the bis(allyl ether) to the corresponding bis(methyl vinyl ether) ##STR4## The Reppe addition reaction cannot be realized economically. For the second reaction, 2-chloroethyl ether is required, the use of which is undesirable for physiological reasons.
It is a disadvantage, common to vinyl ethers, that, in the presence of moisture and traces of acid, these enol ethers split into low-boiling, highly odoriferous carbonyl compounds: ##STR5##